Tailored response microwave filter

ABSTRACT

A multiple section microwave filter capable of being accurately tailored to special frequency response requirements over wide bandwidths in disclosed. The tailored response filter consists of an array of cascaded traveling-wave directional filters. Each filter section couples power out of a through transmission line into suitable microwave terminations. Due to the directional characteristics of traveling-wave directional filters, there is no interaction between sections. Thus the frequency response of the array is the product of the transfer functions of the individual sections. The coupling constants and the center frequencies of individual filter sections are tailored, i.e., synthesized, to result in a set of individual transfer functions which will produce the desired overall response. The tailored response filter can be used in wide bandwidth microwave systems for phase and amplitude weighting and for equalization functions.

United States Patent [72] Inventor Donald R. Wehner 2,762,017 9/1956 Bradburd et al. 333/9 San Diego, Calif. 3,237,134 2/ I966 Price 333/73W [21] i g 1969 Primary ExaminerHerman Karl Saalbach z 6 1971 Assistant Examiner-C. Baraff [73] Assignee The United States of America as represented figigi; warfield George Rubens and John by the Secretary of the Navy ABSTRACT: A multiple section microwave filter capable of [54] TAILORED RESPONSE MICROWAVE FILTER being accurately tailored to special frequency response requirements over wide bandwidths 1n disclosed. The tailored 1 Claim, 4 Drawing Flgs.

response filter consists of an array of cascaded traveling-wave [52] 0.8. CI 333/73, directional filters, Each filter section couples power out of a 333/84 lmofficlal) through transmission line into suitable microwave termina- [51] Int. Cl 03h 7/14, tions, Due to the directional characteristics of traveling-wave H01!) directional filters, there is no interaction between sections. [50] Field of Search 333/9, 10, Thus the frequency response of the array is the product of the transfer functions of the individual sections. The coupling 56 R f constants and the center frequencies of individual filter sec- 1 e erences tions are tailored, i.e., synthesized, to result in a set of in- UNITED STATES PATENTS dividual transfer functions which will produce the desired 3,092,790 6/1963 Leake et al. 333/10 overall response. The tailored response filter can be used in 2,808,573 10/1957 DeBell 333/73C wide bandwidth microwave systems for phase and amplitude 2,922,123 1/ l960 Cohn 333/10 weighting and for equalization functions.

M 29 C 'l '2 l9 2/ 22 ,8 4- C in c c c 5 c 2 2 22-. l 23-. V, Q- 22 Patented April 6, 1971 3,573,574

2 Shasta-Sheet 1 /6J n Q- INVENTOR DONALD 1?. WEHNE'R AT TOR/VEVS Patented April 6, 1971 3,573,674

2 Sheets-Sheet 2 TAILORED RESPONSE l BANDWIDTH ATTENUATION(db) -l ATTENUATION (DB) INVENTOR DONALD R. WEHNER ATTORNEYS TAILORED RESPONSE MICROWAVE FILTER STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of Amer- BACKGROUND OF THE INVENTION Conventional coupled-section filters using Butterworth or Tchebyscheff constants can be used to provide a variety of unique and fixed attenuation versus frequency responses, some of which may be useful as equalization or weighting filters. However, adequate fitting to special requirements is impossible with these techniques. Another problem inherent in existing coupled-section filters is that the time delay through the filters changes rapidly where attenuation increases. In addition, matching problems exist because these filters tend to attenuate by reflection of the input signals rather than by dissipation. As is well known, attenuation by reflection results in high VSWR in regions of high attenuation.

Another approach which is known is the use of a combination of reactances and resistances along a transmission line which attenuate input signals at desired frequencies. This technique allows tailoring but again there is a time delay varia' tion with frequency within the bandwidth.

SUMMARY OF THE INVENTION The invention comprises a tailored response filter which provides amplitude versus frequency response accurately tailored to special requirements over wide bandwidths in the L to X band regions. The filter consists of an array of cascaded traveling-wave directional filters which are located along a transmission line in an energy coupling relationship thereto.

Each filter section couples power out of the through transmission line and into a microwave termination. Each section is matched to the line impedance and attenuates by dissipation. Due to the directional characteristics of traveling-wave directional filters, there is no interaction between sections. Thus, the overall frequency response of the array is the product of the transfer functions of the individual sections.

Coupling constants and center frequencies of the individual filter sections are tailored to result in a set of individual transfer functions which will produce the desired overall response. Actual determination of the coupling constants is conveniently accomplished by means of a computer program which adjusts the constants in an iterative fashion until the desired overall response is achieved.

STATEMENT OF THE OBJECTS OF THE INVENTION An object of the present invention is to provide a microwave filter which can be accurately tailored to special frequency response requirements over wide bandwidths.

Another object of the present invention is to provide a multiple section microwave filter in which the individual sections consist of traveling-wave directional filters, each of which dissipates a selected portion of the bandwidth of an input signal.

Another object of the present invention is to provide a tailored microwave filter in which multiple directional filters are cascaded along a through transmission line in an energy coupling relationship thereto to shape frequency response by means of frequency selective dissipative attenuation.

Another important object of the present invention is to provide db. tailored microwave filter consisting of a plurality of directional filters all of which are matched over a wide band of frequencies but each of which attenuates only a predetermined bandwidth segment of the input signal.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic block diagram of the tailored response filter of the present invention;

FIG. 2 is a stripline representation of a typical directional filter section;

FIG. 3 is a graphical representation of the frequency responses of the individual filter sections and of the overall frequency responses of the tailored filter; and

FIG. 4 is a graphical representation of the response of a typical directional filter as the value of coupling coefficient C is varied while the value of coupling coefficient C, is kept constant.

DESCRIPTION OF THE PRINCIPAL EMBODIMENT FIG. I is a schematic representation of the tailored response filter of the present invention. In the FIG., element 10 represents a through transmission line for transmitting microwave energy. The line 10 has an input port 11 at which input microwave energy is applied and an output port 12 at which output microwave energy is derived. A plurality of directional filters are shown disposed in a cascaded fashion along the same side of line 10 in close proximity thereto.

Each filter section includes a pair of quarter-wave couplers l3 and l7, l4 and l8, l5 and 19, and I6 and 20 as part of resonant loops 21, 22, 23, and 24, respectively. The couplers and loops have center frequencies f0 f0 f0 and f0, respectively.

Couplers l3l6 are disposed in close proximity to each other in a coupling relationship with respect to transmission line 10. That is, there is a mutual coupling relation between the above couplers and line 10 which permits energy transfer from one to another. However, due to the characteristics of directional filters, there is no mutual coupling between couplers I3I6.

The measure of the amount of coupling existing between couplers 13l6 and line 10 is represented by voltage coupling coefficients C2 C2 C and C,,, respectively.

Associated with each of the couplers l3-16 and disposed in a coupling relationship thereto are corresponding directional couplers 17-20. Voltage coupling coefficients C1,, C1 C1 and C1,, represent the measure of coupling existing in couplers l720 which couple energy from loops 21- -24 respectively. Microwave terminations 29 attenuate the energy thus coupled by dissipation.

Again, due to the directional characteristics of travelingwave directional filters, there is no coupling between the various filter sections.

As can be seen from FIG. 1 and FIG. 2, the couplers comprise one-half the length of square loops 21-24. Each side of the loops has a dimension of approximately a quarter-wave length at the center frequency shown in FIG. 1. The loops need not actually be square shaped, but the line length of each must be an integer number of wavelengths long at the center frequency.

Since wavelength is an inverse function of frequency, and since the center frequencies shown increase from a value of f0, to a value of f0,,, it is obvious that the wavelengths of the successive loops and couplers decrease as higher and higher center frequencies are selected. Thus, loops 2I-24 are of progressively different dimensions so that they are resonant to progressively different frequencies. That is, loop 24 is smaller than loop 23 which is smaller than loop 22 which is further smaller than loop 21.

The invention can be made in a number of microwave transmission line modes such as, stripline, coaxial line, waveguide or microstrip.

The number of filter sections which can be used depends upon the bandwidth of interest and upon the center frequencies selected for the individual sections. Thus, although only four sections are shown in FIG. 1, it should be understood that the dashed line between the last two sections indicates that as many sections as are necessary to produce a tailored output can be used.

network consisting of a transmission line loop connected in 10 an energy coupling relationship to directional couplers l4 and 18.

Input microwave energy E,, applied to port 1 travels in transmission line 10 in the direction indicated by the arrow.

Due to the coupling existing between line 10 and quarter-wave directional coupler l4, microwave energy from line 10 is transferred to loop 22. The amount of microwave energy which is coupled to loop 22 is a function of the coupling coefficient C which exists between line 10 and coupler 14. For purposes of explanation, C and C, of FIG. 2 refer to C2 and C1 of FIG. 1, respectively. The energy which is coupled travels in loop 22 in the opposite direction of E as indicated by the arrows in FIG. 2. Resonant loop 22 circulates the coupled energy in a resonating fashion.

The energy which is coupled to loop 22 is in turn coupled out of the loop into termination 29 in an amount determined by coupling coefficient C As before, the energy transferred out of the loop 22 travels in the opposite direction of the energy in the loop, thus the energy E at port 3 equals zero.

Substantially all of the microwave energy E coupled from the through line 10 is dissipated in microwave tennination 29. Thus the output microwave energy E derived at port 2 is approximately equal to E minus the energy E dissipated in termination 29.

The remaining filter sections operate in the same manner so that the input microwave energy applied at input port 11 is successively attenuated by dissipation in the other filter sections. As can be seen from FIG. 3 the individual frequency responses 25-30 are superimposed to produce an overall frequency response curve 31 having a tailored response bandwidth as shown.

The original analysis of the traveling-wave filter was developed by F. S. Coale in a technical publication entitled A Traveling-Wave Directional Filter, Institute of Radio Engineers. Transactions: Microwave Theory and Techniques, v. M'IT-4, p. 256-260, Oct. 1956. Coales original analysis has been expanded for the transfer function from ports 1 to 4 of FIG. 2 for the case where C #0 In the design of a tailored response filter, the transfer function from ports 1 to 2 of the individual sections is of interest. Since ports 3 and 4 are terminated, power coupled from the through line 10 is dissipated in the port 4 termination and the power P into port 3 termination is 0, thus, P +P =P so that 4 By substituting in equation (I) and expressing the power ratio in decibels, it can be seen that Eggs... ..g 0.20..

cos 3+ (1 l-a) sin If the effect of line loss is neglected, the quantity db approaches infinity at F,,, and the response narrows as C and C approach the same value as can be seen in FIG. 4.

The overall frequency response in db. attenuation versus frequency of the cascaded sections is the summation of the individual responses given by Equation (2) as can be seen from FIG. 3. The desired overall attenuation at some frequency f can be denoted by a (j) and the calculated attenuation of each filter section by A( F) where For example, assume an N element filter with a trial set of N resonant frequencies, f0 and N pairs of coupling constants C and C The range of frequencies of interest can be divided into M discrete test frequencies, f,,,,=f,-f at which the overall response is to be tested. An array of error terms is then generated as follows:

The first term is residual attenuation calculated at the frequency f for which the desired attenuation is minimum or zero.

Reasonably rapid convergence results by selecting N of the M test frequencies to be the N resonant loop frequencies and using theresulting error terms to adjust one of the coupling constants, C or C in an iterative fashion. When C is chosen to be fixed, the convergence criteria are as follows:

where AC is the incremental change in the coupling used for iteration.

By means of an iterative computer program, Equation (3) can be solved automatically, and the convergence criteria of Equation (4) can be applied automatically also. In such a program, the user selectsa trial number of filter sections, N, with their resonant loop frequencies F trial coupling constants, C and increment, Af, between test frequencies, and the Thus, it can be seen that a new and novel microwave filter capable of being accurately tailored to special frequency response requirements over wide bandwidths has been disclosed. Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Iclaim:

l. A microwave tailored response filter for use over a wide bandwidth comprising:

a. a through transmission line having an input port and an output port for receiving and transmitting microwave energy;

b. an array of traveling-wave directional filters each having a different center frequency;

c. each of said directional filters consisting of a resonant loop, said resonant loop having associated therewith first and second quarter-wave directional couplers, said loop and said couplers both having the same center frequency within the bandwidth of interest;

d. said first directional couplers being disposed and arranged in a cascaded manner along said transmission line in an energy coupling relationship thereto;

e. said first directional couplers coupling a predetermined amount of energy out of said transmission line into its corresponding resonant loop, said amount being a function of the coupling coefficient C existing between said first directional couplers and said transmission line;

each resonant loop circulating the energy within it in a resonating fashion;

g. each of said second directional couplers being disposed and arranged in a cascaded manner and in an energy coupling relationship with respect to its corresponding resonant loop;

h. each of said second directional couplers coupling a predetermined amount of energy out of its corresponding resonant loop, said amount being a function of the coupling coefficient C existing between each of said second directional couplers and its corresponding resonant loop; and V a attenuation means associated with each of said second directional couplers for dissipating the energy coupled into said coupler. 

1. A microwave tailored response filter for use over a wide bandwidth comprising: a. a through transmission line having an input port and an output port for receiving and transmitting microwave energy; b. an array of traveling-wave directional filters each having a different center frequency; c. each of said directional filters consisting of a resonant loop, said resonant loop having associated therewith first and second quarter-wave directional couplers, said loop and said couplers both having the same center frequency within the bandwidth of interest; d. said first directional couplers being disposed and arranged in a cascaded manner along said transmission line in an energy coupling relationship thereto; e. said first directional couplers coupling a predetermined amount of energy out of said transmission line into its corresponding resonant loop, said amount being a function of the coupling coefficient C2n existing between said first directional couplers and said transmission line; f. each resonant loop circulating the energy within it in a resonating fashion; g. each of said second directional couplers being disposed and arranged in a cascaded manner and in an energy coupling relationship with respect to its corresponding resonant loop; h. each of said second directional couplers coupling a predetermined amount of energy out of its corresponding resonant loop, said amount being a function of the coupling coefficient C1n existing between each of said second directional couplers and its corresponding resonant loop; and i. attenuation means associated with each of said second directional couplers for dissipating the energy coupled into said coupler. 